The IC engine, Otto or Diesel cycle piston engine, or rotary engine, have been in continuous development and improvement since the nineteenth century and have maintained a dominant position as powerplants for machines and vehicles. These engines range in output from a fraction of a horsepower to tens of thousands of horsepower.
The simplest IC engine, like the ones used in some model aircraft, can have a single piston, a single connecting rod, a crankshaft and a simple jet carburetor contained in a simple cylinder and crankcase structure. No ignition system is necessary if self igniting fuel, such as for example, diesel cycle fuel, is used. Such an air-cooled 2-cycle engine does not have all the valvetrain, lubrication system and liquid cooling systems of the common automotive engines.
In order to achieve high fuel efficiency, low exhaust emissions and smooth operation at a wide range of power levels, rotational speeds and environmental conditions, the modern IC engine deviates from simplicity and uses an increasing number of engine subsystems, sensors and controls. For example, a current design automobile engine includes the following:                a. Multiple cylinders (4-12) for smooth operation, which multiplies the pistons, rings, connecting rods, etc.        b. 4-cycle valvetrain with 4-valves per cylinder becoming more popular        c. Fuel injection system (pump, injectors, pressure regulator, etc.)        d. Lubrication system (pump, pressure regulator, filter, etc.)        e. Liquid cooling system (pump, radiator, temperature control)        f. Throttle and ignition system in an Otto cycle engine        g. Cold start system in a Diesel cycle engine        h. Turbocharger or supercharger and intercooler in some engines        i. Sophisticated engine control system which includes computer(s), intake air density sensors and in some cases exhaust gas sensors, variable intake manifold control, exhaust gas re-circulation system, etc.        
The large number of subsystems, sensors, electrical wiring, liquids plumbing and controls increases the modes of possible failure of the modern IC engine. In automotive use, a very long and expensive development and testing cycle improves the subsystem reliability to a level acceptable for automotive use. In the vast majority of automotive cases an engine failure is not a safety issue and the expansive development and testing cycle is affordable for the largest automotive markets. But, in some specialty sports car markets engine reliability is a substantial problem, if the engine is not common to that of another car in large production.
In the aircraft industry, the significant chances of failures of the engine subsystems has caused the major aircraft markets to use turbine engines and the low-cost end of the aircraft market to use the reasonably reliable very simple (air-cooled, 2-valve per cylinder, mostly with carburetor and without turbocharger) engines developed in the 1930's and 1940's.
The aircraft is probably the best example where the combination of requirements of very high reliability, high performance and development cycle affordable for a low production rate caused the IC engine to be almost completely replaced by the turbine engine. Aircraft engine reliability is critical to flight safety especially in a large passenger aircraft because of long range flight over water or rough terrain, low chance of survival due to high glide speeds and lack of ejection seats common to fighter aircraft. The flight safety of passenger aircraft dramatically improved since the 1950's when the piston engine was rapidly replaced by the much simpler and more reliable turbine engine, even when four-engine aircraft were replaced by two-engine aircraft.
Currently, only the lowest cost aircraft for personal use and for training use are powered by IC engines. The most commonly used such IC engines were developed in the 1930's and 1940's, are air-cooled and very simple as compared to a modern car engine. The low number of subsystems in such engines and the improvement cycle of the last 50-60 years provided better reliability at the expense of lower performance and operational refinements.
In order to improve operational safety, both aircraft and ships use multiple engines. This propulsion redundancy makes the operation substantially more complex and in the case of an aircraft necessitates special training and pilot certification for multi-engine operation. In the case where the multi-engine propulsion is required to be coupled to a single driven device, as for example, the rotor system of a helicopter, a system of coupling and clutching is used to provide for operation of the rotor with one engine inoperative.